The present invention relates to a valve unit and a method for controlling a biasing force acting on a valve element, and more particularly to applying a pressurized fluid to a control passage via a control unit, in order to bias the valve element to a first position. In particular, the invention relates to a valve unit of a fuel injector.
One such fuel injector is for example shown in U.S. Pat. No. 5,833,146 filed on Nov. 10, 1998 which is assigned to the assignee of the present invention. In the known fuel injector, fuel is pressurized to a high pressure via an intensifier piston and then supplied to a nozzle chamber in which a needle valve member reciprocates for opening and closing an injection nozzle. The needle valve member has a hydraulic surface on which the pressurized fuel acts. The surface is arranged such that the fuel applies a force to the needle valve member, thereby moving the needle valve in an opening direction. The needle valve member is biased in a closing direction by a compression spring. The pressure which is applied to the needle valve member by the fuel is capable of moving the needle valve member in the opening direction against the biasing force of the compression spring.
The needle valve member is also biased in the closing direction by a pressurized actuation fluid which acts on a piston contacting the needle valve member. The pressurized actuation fluid is applied to the piston which is arranged in a piston chamber of the injector body via a control line and a pilot valve. The cross sectional area of the piston surface, on which the pressurized actuation fluid acts is substantially larger than the cross sectional area of the surface on the needle valve member on which the pressurized fuel acts. Thus, a relatively low actuation fluid pressure may hold the needle valve member in a closed position against the force of the high pressure fuel acting on the needle valve. The actuation fluid therefore controls opening and closing of the needle valve, since the fuel pressure acting on the needle valve is usually not capable of moving the needle valve against the pressure applied to the piston. The pilot valve is arranged at an opposite end of the fuel injector with respect to the needle valve member due to space restrictions.
In order to start an injection event, the pressure acting on the piston is lowered via the control line and the pilot valve. Due to the long length of the control line which acts as a throttle and a small cross sectional area of the exit opening at the pilot valve, the venting of the actuation fluid is slow and takes a long time. After reaching a predetermined pressure of the actuation fluid at the piston, the fuel pressure acting on the needle check valve is high enough to move the needle valve member in the opening direction to thus start injection of fuel. Closing, the needle valve member is accomplished by applying pressure on the piston via the pilot valve and the control line. If the fuel pressure falls below a certain level during injection, the needle valve member can also be closed by the biasing force of the spring.
In order to have accurate control of the injection events in an engine, it is important that the needle valve member is quickly opened. In the known injector this is not possible due to the long control line length and the small cross sectional area of the outlet opening in the pilot valve.
The present invention is directed to overcoming one or more of the problems as set forth above.
In one aspect of the present invention a valve unit has a needle valve member, a control valve, a control line between the control valve and the needle valve member, wherein pressurized actuation fluid may be applied to the needle valve member via the control valve and the control line, in order to bias the needle valve member in a first direction, a vent passage which is connected to the control line between element needle valve member and the control valve and a valve element which is movable in the control line between a first position in which said vent passage is closed, and a second position in which said vent passage is open to the control line.
In another aspect of the present invention a fuel injector has an injector body defining a nozzle chamber having at least one injection nozzle, a hydraulic unit for pressurizing fuel in a the nozzle chamber, a needle valve member, which is movable between a first position in which the at least one injection nozzle is closed and a second position in which the at least one injection nozzle is open, a control valve, a control line extending between the needle valve member and the control valve, wherein pressurized fluid may be applied to the control line via the control valve, for biasing the needle valve member to its first position, a vent passage, which is connected to the control line between the needle valve member and the control valve, and a valve element, which is disposed in the control line and is movable between a first position blocking the fluid connection between the vent passage and the control line, and a second position opening fluid connection between the vent passage and the control line. The position of the valve element is controllable by pressure applied to the control line via the control valve.
In yet another aspect of the invention, a method for controlling the biasing force acting on needle valve member is provided. The method comprises the following steps: applying a pressurized fluid to the needle valve member via a control valve and a control line arranged between the control valve and the needle valve member for biasing the needle valve member in a first direction, venting at least part of the fluid via the control valve and lowering the biasing force, venting at least part of the fluid via a vent passage which is fluidly connected to the control line between the control valve and the needle valve member, the connection being opened by a valve element disposed in the control line in response to said partial venting of the pressurized fluid via the control valve.